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Genetics is obviously an important cause of whether someone becomes extraverted or introverted. But what percentage is genetics and what percentage is the environment? What factors in the environment are most important in determining this trait?

It may seem like the definitions of introverted and extroverted are clear enough, in reality I don't think they are. Do you mean the way I would define it: Does a person re-energize by being alone and doing their own thing (introvert) or do they re-energize by socializing (extrovert). OR do you mean a person is shy and quiet versus outgoing and/or talkative. In the first definition, I think that is all genetics. In the second definition it is almost all environment.
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DunkJan 31 '12 at 15:28

@Dunk: Interesting point. I mean the first one as that appears to be the psychology definition
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CasebashJan 31 '12 at 21:13

@Casebash-I don't know any studies, just my personal experience with my kids and can certainly say that within the first few weeks/months you can definately tell what the child's personality is going to be. So it has to be genetics. We have one very, very outgoing child and one far more reserved. We have worked hard at getting the reserved one to be sociable and it has been working but I know it is always going to feel like work to him, throughout his life. While for the sociable one, that is what he likes to do. That is his fun.
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DunkJan 31 '12 at 21:54

1 Answer
1

Heritability estimates of Extraversion (and other Big 5 factors)

The introduction section of Loehlin et al (1998) provides a narrative review of heritability estimates of big 5 personality traits (i.e., one of which is extraversion).
A brief extract gives a flavour of some of the research that has been conducted:

A recent heritability analysis of the Big Five dimensions as measured
by the Revised NEO Personality Inventory of Costa and McCrae (1992)
used a total of 660 monozygotic (MZ) pairs and 380 dizygotic (DZ)
pairs from pooled Canadian and German twin samples (Jang, McCrae,
Angleitner, Riemann, & Livesley, 1998). For all five traits a simple
model involving only additive genes and nonshared environment fit the
data. Estimates for the heritabilities of factor scales for [Extraversion, Agreeableness, Conscientiousness,
Neuroticism, and Openness] were .50, .48, .49, .49, and .48, respectively. These
estimates are obviously very similar—in partic- ular, the heritability
of Agreeableness is quite in line with the rest.

They also mention other twin studies, with heritability estimates for Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness respectively as:

Thus, in summary, it seems that such estimates for the heritability of extraversion are around 50%.

Side Point

While inventories like the Myer-Briggs Type Inventory often categorise individuals, into categories, I find a continuous approach to personality traits provides a more accurate description of an individual. Using such an approach, people have a score on an extraversion dimension that is roughly normally distributed (or at least continuously distributed) in the population.

Limitations

I realise this only partially answers your question. It doesn't discuss the dynamic environmental-genetic interactions that cause personality traits , nor does it take a critical perspective on issues related to the measurement of extraversion, and the estimation issues related to heritability. Hopefully, it is a useful starting point.

Loehlin, J.C. and McCrae, R.R. and Costa, P.T. and John, O.P. (1998). Heritabilities of common and measure-specific components of the Big Five personality factors. Journal of Research in Personality, 32, 4, 431-453. FREE PDF